Surface plasmon resonance (SPR) is a surface optical technique which is sensitive to the thickness and index of refraction of material at the interface between a noble metal (e.g. gold, silver, or copper) and a bulk medium, such as air or water. Surface plasmon resonance may be achieved by using the evanescent wave which is generated when a laser beam linearly polarized parallel to the plane of incidence (p-polarized) impinges onto a prism coated with a thin metal film. The metal may also be coated onto a thin transparent substrate such as glass, and this glass brought into optical contact with the prism. SPR is most easily observed as a reduction of the totally internally reflected light just past the critical angle of the prism. This angle of minimum reflectivity (denoted as the SPR angle) shifts to higher angles as material is adsorbed onto the metal layer. The shift in the angle can be converted to a measure of the thickness of the adsorbed or added material by using complex Fresnel calculations and can be used to detect the presence or absence of materials on top of the metal layer.
In using SPR to test for biological, biochemical or chemical substances, a beam of light from a laser source is directed through a prism onto a biosensor consisting of a transparent substrate, usually glass, which has one external surface covered with a thin film of a noble metal, which in turn is covered with an organic film that interacts strongly with an analyte, such as a biological, biochemical or chemical substance. The organic film can contain substances, such as antibodies or antigens, which can bind with an analyte in a sample to cause an increased thickness which will shift the SPR angle. By either monitoring the postition of the SPR angle, or the reflectivity at a fixed angle near the SPR angle, the presence or absence of an analyte in the sample can be detected.
Various types of equipment for using SPR with a biosensor for biological or biochemical or chemical substances are disclosed in the Liedberg, et al. article in Sensors and Actuators, Vol. 4, 1983, page 299; European Patent Application 0305108; and, the recently issued Maule U.S. Pat. No. 5,374,563.
The use of SPR as a testing tool offers several advantages; it is fast, it requires no labeling and it can be done on site. However, to fully achieve these advantages there is a need for a simple, practical biosensor which can be readily modified or adapted to test for a wide variety of analytes, including, biological, biochemical or chemical substances.